Photonic integrated circuits: where are the limits?

The integration scale in Photonic Integrated Circuits will be pushed to VLSI-level in the coming decade. Key technologies for reduction of device dimensions are high resolution lithography and deep waveguide etching technology. In this paper developments in Photonic Integration are reviewed and the limits for reduction of device dimensions are discussed. ©2004 Optical Society of America OCIS codes: (130.3120) Integrated optics devices, (130.5990) Integrated Optics: Semiconductors Introduction. The title that the IPRA Programme Committee suggested for this presentation was: “Photonic integrated circuits with deeply etched waveguides”. Deep etching causes a strong lateral confinement of light. Deep etched waveguides can be narrower and, what is more important, they show much lower bending loss than shallow etched waveguides. Even with a very small radius (< 10 μm) they can be used for low-loss interconnection. This allows for a strong reduction of the size of interconnection circuits. Further, deep etched MMI-couplers and deep etched AWG’s, key components in Photonic Integrated Circuits, can be made much smaller than shallow etched ones. Deep etching seems to be the key to reduction of device dimensions and the question raises: where are the limits? This will be the subject of the present paper, and we changed the title accordingly. The dynamics of micro-electronic integration technology. The field of Information and Communication Technologies is showing a development speed which is unprecedented in history. Over a period of more than thirty years key features like processor speed and memory size are roughly doubling each 18 months, and experts believe that this development will continue in the coming decade. It is known as Moore’s law and enabled by the developement of micro-electronic integration technology. Due to this development a steadily increasing performance of components and systems can be offered at an essentially constant price, a development that we recognise in the fact that the price of our new PC does not differ much from what we paid three years earlier for the previous one. This dynamics has become a major driver for the ICT market, which would collapse without this steady innovation. Moore’s law in micro-photonics. If Photonics is going to play a substantial role in the ICT market it will have to follow the same dynamics. If not, it will rapidly loose the competition with micro-electronic solutions. The increase in functionality prescribed by Moore’s law can only be sustained by applying an integration technology that supports a steady reduction of circuit size and fabrication costs. As such Indium-Phosphide based integration technology is the most powerful technology because it supports the integration of almost all functions required in ICT applications. The question is whether it has the same potential as micro-electronic integration technology for reduction of device dimensions and fabrication costs over a longer period. To study that question we have put the development of Photonic IC’s in our own lab in a graph with the potential integration density in devices per square centimeter on the vertical axis. The open circles mark the first publication of a device or a circuit. It starts with the invention of the AWG in 1988 [1]. The next circle is the first InP-based Optical Add-drop Multiplexer (OADM) by Vreeburg in 1997 [2], a device that integrated a single AWG with four Mach-Zehnder switches on an area of 0.2 cm; 25 components per square centimeter. As a next step we developed a technology for reducing the size of our AWG’s using deep etching technology. This brought us the worlds most compact Optical Cross-Connect (OXC) [3]: a device with 4 AWG’s and 4 Mach-Zehnder switches on an area of 5 mm, i.e. an integration density of more than 100 components per square centimeter. Since then we succeeded in a further reduction of AWG-size close to the limits of conventional deep etched waveguide technology in InP: 250x350 μm [4]. The last circle in the graph is a photonic flip-flop, consisting of two deep-etched micro-ring lasers, that was recently published by Hill et al. [5]. This device, with dimensions of 20x40 μm, allows for integration of more than 1000 components per square centimeter, in principle. As can be seen these devices fit remarkably well to a straight line with a slope slightly larger than Moore’s law. If this is significant it confirms the observation that the dynamics of telecommunications is even faster than that of micro-electronics. IWB1